The present invention relates to maraging steel and a method of producing the same.
Since maraging steel has a very high tensile strength of around 2000 MPa, the maraging steel has been used for members, which are required to have high strength, such as those for rockets, centrifugal separators, aircraft, and continuously variable transmissions of automobile engines, tool, die, and so on.
The maraging steel usually contains, as strengthening elements, appropriate amount of Mo and Ti, so that the maraging steel can have high strength, which is achieved by such an aging treatment as to precipitate intermetallic compounds such as Ni3Mo, Ni3Ti, and Fe2Mo. A typical maraging steel containing Mo and Ti has a chemical composition of, by mass percent, 18% Ni, 8% Co, 5% Mo, 0.45% Ti, 0.1% Al and the balance of Fe.
However, while the maraging steel can have very high tensile strength, fatigue strength thereof is not necessarily high. The most notabilian factors deteriorating fatigue strength of the maraging steel is non-metallic inclusions of nitride and/or carbonitride such as TiN and TiCN. When the non-metallic inclusions coarsen in the steel, fatigue fracture is initiated from the inclusions.
Thus, in general, in order to reduce the non-metallic inclusions in the steel, a vacuum arc remelting process (hereinafter referred to as VAR) has been used.
The maraging steel produced by the VAR process has advantages that it is homogeneous (i.e. small segregation) and that the amount of non-metallic inclusions is reduced.
However, comparatively large non-metallic inclusions of nitrides or carbonitrides such as TiN and TiCN also remain in the maraging steel produced by the VAR process. The residual large non-metallic inclusions still exist in the material after hot forging, heat treatment, hot rolling, and cold rolling which are performed after VAR. This has been a cause of fatigue fracture initiated from large residual non-metallic inclusions.
In order to solve the problem, various proposals have been made. For example, in JP-A-2001-214212, a method of producing Ti-containing steel is disclosed, according to which a raw material of Ti-containing steel without titanium-nitride inclusions is melted in a vacuum induction furnace, and cast to produce a Ti-containing steel material as an electrode, and the material is re-melted in a vacuum arc melting process to refine the titanium-nitride inclusions.
The present inventors have studied further enhancement of cleanliness of the maraging steel.
In the JP-A-2001-214212, the raw materials for Ti-containing steel, which do not contain nitride inclusions such as TiN and TiCN can be used to refine titanium-nitride inclusions. This management of qualities of the raw materials is one measure for reducing the nitride-base non-metallic inclusions, but there is a problem that a high-grade raw material is naturally an expensive raw material and cost is high.
Moreover, since the generation of the titanium-nitride inclusions also depends on melting conditions, the problem cannot sufficiently be solved only by the management of the raw materials.
Additionally, the maraging steel has a very high tensile strength of around 2000 MPa, but the fatigue fracture caused from the residual non-metallic inclusions which are the fracture origin in a high fatigue region exceeding 107 times has raised a problem. Especially, when the maraging steel is formed into a thin strip, there is a high possibility of breakage of the thin strip by propagation of fracture of the non-metallic inclusions.
The fatigue fracture by the non-metallic inclusions is determined by the size of the non-metallic inclusion. When the maraging steel is applied to a thin strip material, the presence of the non-metallic inclusion itself raises a large problem with the use in the high fatigue region exceeding 107 times.
Further, actually, in the maraging steel, oxide inclusions are also confirmed in addition to nitride inclusions. The number of existing oxide inclusions is small, but the inclusions having a comparatively large size, for example, a diameter exceeding 20 μm, are sometimes confirmed.
There is a concern about that the presence of such large oxide inclusions adversely affect mechanical characteristics of the material such as the fatigue strength like as in the case of nitride inclusions such as TiN.
The examples of the method of reducing the non-metallic inclusions caused by gas components such as nitride and oxide include vacuum remelting processes such as VAR, but there is a limitation onto the reduction of the size of the nitride or oxide inclusion only with the application of VAR. Therefore, there has been a strong demand for development of a new breakthrough technique which is remarkably effective in reducing the size of the non-metallic inclusion of the maraging steel.